Before his death in 2004, an ill Manchester asked former Cox newspapers journalist Reid to take his research notes and finish writing the final volume of his trilogy. The long-delayed majestic account of Winston Churchill’s last 25 years is worth the wait. Sixty-five when he became Britain’s prime minister in 1940, Churchill remained a Victorian aristocrat, self-indulgent, coddled by servants. Yet his vitality, charisma, and self-assurance made him a perfect leader in a crisis. During his first year, when Britain fought Nazi Germany alone, Churchill, say the authors, may have saved civilization. Once the U.S.S.R. and U.S. joined, Britain’s role declined but not Churchill’s energy. While FDR left war to his generals, Churchill poured out ideas, many of them imaginative failures (the bloody landing at Anzio) or simply bad (early opposition to invading France). Despite Churchill’s unparalleled popularity, his Conservative party was defeated in July 1945. Though devastated, Churchill remained the party leader, returning to office in 1951 to preside over a declining empire and escalating cold war until a repeatedly postponed retirement in 1955. Manchester (and Reid) matches the outstanding quality of biographers such as Robert Caro and Edmund Morris, joining this elite bank of writers who devote their lives to one subject. 32 pages of b&w photos, 6 maps. (Nov.)